Sulfate improves cadmium tolerance by limiting cadmium accumulation, modulation of sulfur metabolism and antioxidant defense system in maize

Abstract Cadmium (Cd) contamination of agricultural soil has become a serious threat to global food security. The present study highlights the role of added sulfate (SO 4 2− ) in modulating sulfur metabolism and antioxidant defense system conferring tolerance against Cd stress in maize. Expression patterns of antioxidant genes, transporters involved in SO 4 2− and Cd accumulation, antioxidant enzyme activity, membrane damage, in vivo reactive oxygen species (ROS) detection and hydrogen peroxide (H 2 O 2 ) accumulation under Cd stress in presence (T 2 ) or absence (T 1 ) of excess SO 4 2− were studied to get an overview of cellular manoeuvering in conquering Cd induced oxidative stress damages. Moreover, dynamic correlations between miR398a, miR395d, miR408a and their target genes under Cd exposure were validated through qRT-PCR. Supplementation of the ¼ Murashige and Skoog (MS) media with excess sulfate [600 μM (NH 4 ) 2 SO 4 ] markedly restored the shoot biomass under Cd stress (100 μM CdCl 2 ). Presence of excess SO 4 2− in the nutrient media significantly reduced Cd uptake as well as tissue Cd accumulation in T 2 plants. Foliar dark blue and deep brown spots revealed after histochemical staining with nitroblue tetrazolium (NBT) and 3′3′-diaminobenzidine (DAB) respectively indicated severe oxidative burst in T 1 plants under Cd treatment. The chloromethyl derivative of 2′, 7′-Dichlorofluorescin diacetate (CM-H2DCFDA) and dihydroethidium (DHE) staining further supported enhanced ROS formation in roots of Cd challenged T 1 plants. Enhanced reduced glutathione (GSH) level both in root and above ground part of T 2 plants might be responsible for their better performance under Cd stressed condition as evident from non-significant increase in superoxide anion (O 2 − ), low H 2 O 2 and thiobarbituric acid reactive substances (TBARS) levels as compared to T 1 plants. Taken together, our findings indicate that fine adaptation of sulfate uptake and assimilation in sulfate supplemented maize plants (T 2 ) might satisfy two contrasting needs: (a) maintaining high GSH pool essential for sustaining balanced redox status under stress condition; (b) alleviating Cd stress effects by means of GSH mediated detoxification pathways.